Technical Field
The present invention relates to a glass substrate for mask blanks for use in various kinds of lithography, and to its production method. The present invention is favorable for producing a glass substrate for mask blanks to be used in lithography using EUV (extreme ultraviolet) light (hereinafter abbreviated as “EUVL”) (hereinafter this is abbreviated as “glass substrate for EUVL mask blank”).
The present invention is also favorable for a glass substrate for mask blanks for use in lithography using an already-existing transmission optical system, for example, for a glass substrate for mask blanks for lithography using an ArF excimer laser or a KrF excimer laser, and for its production.
Background Art
With the recent tendency toward high-density and high-precision ultra-LSI devices, the specifications required for the surface of the glass substrate for mask blanks for use in various kinds of lithography are becoming severer year by year. In particular, with the wavelength of the light from the exposing source being shorter, the requirements for the profile accuracy of the substrate surface (flatness) and for the absence of the defects (particles, scratches, pits, etc.) in the surface are becoming severer, and a glass substrate having an extremely high flatness and having few microdefects is desired.
For example, in a case of lithography using an ArF excimer laser as the light from an exposing source, the necessary flatness of the glass substrate for mask blanks is 0.25 μm or less and the necessary defect size in the glass substrate is 0.07 μm or less; and further in a case of a glass substrate for EUVL mask blanks, the necessary flatness of the glass substrate is 0.03 μm or less as the PV value, and the necessary defect size is 0.05 μm or less.
Heretofore in producing glass substrates for mask blanks, a precision polishing method has been proposed for reducing the surface roughness (for example, see Patent Document 1).
The precision polishing method shown in Patent Document 1 is a method for final polishing using colloidal silica after polishing using an abrasive that includes cerium oxide as the main ingredient. In a case where glass substrates are polished according to the polishing method, a batch-type double side polisher in which plural glass substrates are set and both surfaces thereof are polished simultaneously is generally used.
However, in the above-mentioned precision polishing method, the limit of the flatness of the glass substrates obtainable stably is about 0.5 μm as the PV value, since the flatness is influenced by the mechanical accuracy of the carrier to hold the glass substrate, the surface plate to pinch the glass substrate, the planetary gear mechanism to drive the carrier and others.
Recently, therefore, there has been proposed a planarization method for glass substrates through plasma etching or local processing with a gas cluster ion beam (for example, see Patent Documents 2 and 3).
The planarization method shown in Patent Documents 2 and 3 is a planarization method for the surface of a glass substrate, which includes measuring the roughness profile of the surface of a glass substrate and locally processing the protrusions in the surface under the processing condition (plasma etching amount, gas cluster ion beam amount, etc.) in accordance with the protruding degree of the protrusions. In the method, with the protruding degree relative to the desired profile in each protrusion site increasing more, that is, with the processing amount in each protrusion site increasing more, the moving speed of the local processing tool must be controlled to be lower in order to increase the processing amount therein. In other words, the processing amount and the moving speed of the local processing tool are in an inverse relationship therebetween.
In a case where the flatness of the surface of a glass substrate is controlled through plasma etching or local processing with a gas cluster ion beam, the surface of the glass substrate would be roughened or an affected layer would be formed owing to the local processing, and therefore after the local processing, short-term final polishing would be necessary for improving the surface roughness and for removing the affected layer.    Patent Document 1: JP-A-64-40267    Patent Document 2: JP-A-2002-316835    Patent Document 3: JP-A-8-293483